Sound in a time-dependent ocean: Implications on acoustic tomography of currents

Abstract

Ocean is a nonstationary acoustic environment. Surface and internal waves, tides, and mesoscale eddies contribute to sound-speed and ocean surface variability on different temporal scales. Acoustic effects of the nonstationarity include violation of the reciprocity principle and signal frequency variation along the propagation path. Mathematical models of underwater sound propagation often either ignore ocean nonstationarity or account for it within the frozen-medium approximation. In this paper, accuracy of different approaches to model sound propagation in time-dependent ocean is analyzed within the frameworks of the ray theory and the adiabatic mode approximation. The theoretical approach is based on the space-time geometrical acoustics and a space-time version of the ‘‘vertical modes–horizontal rays’’ technique. While the frozen-medium approximation proves to be generally rather crude and inadequate in modeling nonreciprocity due to sound-speed time dependence, another simple technique, the quasistationary approximation, is shown to be a sufficiently accurate and efficient approach to modeling low-frequency underwater sound propagation. Contributions to ray travel time and mode-phase nonreciprocity due to medium motion and time dependence are compared for several typical current tomography scenarios. The feasibility of distinguishing between acoustic nonreciprocities due to currents and due to medium nonstationarity is discussed. [Work supported by NRC.]